The multiplication of animal cells in vitro requires the presence of macromolecular growth factors in the culture medium. Serum has normally been the source of these multiplication-stimulating factors. Due to the complexities of serum, the mechanism by which these factors stimulate the growth of cells remains obscure. In these proposed studies, a growth factor called multiplication-stimulating activity (MSA) will be purified and used to investigate parameters of cell growth regulation. MSA is produced by a line of rat liver cells which grow in the complete absence of serum. Conditioned medium from these cells will be used as the source for the purifcation of MSA. Purified MSA is a protein of about 10,000 MW which stimulates DNA synthesis and growth of chicken embryo fibroblasts, and has non-suppressible insulin-like activity (NSILA) and resembles the somatomedin family. The ability of purified MSA to regulate the cell cycle in normal and virus-transformed chicken embryo fibroblasts will be examined. Biochemical parameters of membrane function will be monitored in both types of cells as well as the ability of the cells to bind purified labeled MSA, the long-term goal of these studies is to provide insights into the regulation of cell proliferation and to define the biochemical nature of the growth advantage which malignant cells exibit over normal cells.